The present invention relates to an amplifier circuit. More particularly, the invention relates to an amplifier circuit having an output power amplifying stage having transistors connected in an SEPP (Single-Ended Push-Pull) emitter follower configuration.
An amplifier circuit having a power amplifying stage of the Class B SEPP type employing an emitter follower configuration and utilizing bipolar transistors has been known. This circuit suffers from a problem of distortion in the output signal due to the output stage of the circuit operating as an emitter follower circuit. The reasons for this will be described in detail with reference to FIG. 1.
Assuming that the base current of the emitter follower transistor Q.sub.1 is represented by i.sub.b, the base-emitter voltage of the transistor Q.sub.1 is represented by V.sub.BE, and the current amplification factor thereof is represented by h.sub.fe, the following equation may be written: EQU e.sub.o +V.sub.BE +R.sub.g .multidot.i.sub.b =e.sub.i, (1)
where, e.sub.o and e.sub.i represent output and input voltages of the circuit, respectively, and R.sub.g represents its input resistance. Since i.sub.b may be expressed as: ##EQU1## equation (1) can be rewritten in the form: ##EQU2##
From this equation, it is apparent that the factors which can contribute to distortion of the output e.sub.o are nonlinearities in the terms V.sub.BE and h.sub.fe. In equation (3), V.sub.BE cannot be zero, but the righthand term of the denominator can theoretically be reduced to zero by making the input resistance R.sub.g =0. However, it is of course impossible to realize R.sub.g =0, and in an ordinary SEPP circuit, a relation of: ##EQU3## holds. Any attempt to realize R.sub.g &lt;&lt;R.sub.L (1+h.sub.fe) by, for instance, increasing the number of stages in a Darlington circuit has the restriction that the output load impedance R.sub.L of the circuit will be made very low. As a consequence, the output of the SEPP circuit of the emitter follower type is most strongly distorted by nonlinearities in the term h.sub.fe. Hence, elimination of such distortion has been strongly desired.
The input impedance Z.sub.i of an emitter follower circuit can be expressed as: EQU Z.sub.i =h.sub.ie +R.sub.L (1+h.sub.fe), (4)
where, h.sub.ie represents the input resistance of the transistor Q.sub.1 with the emitter grounded. The principal factors which can cause distortion of Z.sub.i are h.sub.ie and h.sub.fe. Although h.sub.ie varies with current, the impedance Z.sub.i is principally defined by R.sub.L and h.sub.fe since h.sub.ie &lt;&lt;R.sub.L (1+h.sub.fe). Among these, h.sub.fe is nonlinear as described above, and in a case where an actual load such as a loudspeaker is utilized as R.sub.L, the input impedance Z.sub.i inevitably exhibits not only a significant nonlinearity, but also a wide phase variation. Since the input impedance Z.sub.i acts as a load on the preceding stage of the overall circuit, variations of the input impedance Z.sub.i cause severe distortion in the preceding circuit. Furthermore, if negative feedback is used in the circuit, the transient characteristics of the circuit are degraded.
Accordingly, an object of the present invention is to provide an amplifier circuit in which distortion present in an ordinary SEPP circuit of the emitter follower type is minimized, and undesirable effects of the load R.sub.L on the input impedance Z.sub.i of the SEPP circuit are eliminated, thereby improving the transient response characteristics of the circuit.